The present invention relates to an automatic choke to be incorporated in a carburetor for use with internal combustion engines, and, more particularly, to an improvement of an automatic choke having a tamper-proof structure.
An automatic choke incorporated in a carburetor of an internal combustion engine generally comprises a choke valve shaft, a casing which is mounted to the body of the carburetor and rotatably supports said choke valve shaft, a cover mounted to said casing so as to define a chamber in co-operation with said casing, a cover mounting means such as a set of mounting ring and screws which mounts said cover to said casing in a manner such that said cover is adjustable relative to said casing with respect to its rotational position around the axis of said choke valve shaft, and a spiral bimetal element disposed in said chamber and having one end connected with said cover and the other end connected with said choke valve shaft. The spiral bimetal element contracts or expands corresponding to the warming-up condition of the engine and operates to drive the choke valve shaft so as to close the intake bore of the carburetor by a choke valve supported by the choke valve shaft when the engine is in the cold state and gradually to open the intake bore by turning the choke valve as the engine is warmed up. Since the performance of such a choking action performed by the automatic choke has a great influence on the startability,, drivability, fuel consumption and exhaust gas emission of the engine, the initial mounting of the spiral bimetal element must be strictly controlled to satisfy the required mounting condition, and it is also important that the initial adjustment in the mounting of the spiral bimetal element should be incapable of being disturbed during vehicle maintenance. On the other hand, it is practically impossible to manufacture every spiral bimetal element strictly to the same dimensions. Therefore, conventionally, the deviations of individual spiral bimetal elements from the standard dimensional condition are compensated for by rotational adjustment of the cover relative to the casing in the process of mounting the cover to the casing by employing a cover mounting means such as mentioned above which permits such rotational adjustment. In this case, the adjustment is generally performed in a manner such that a particular spiral bimetal element is first mounted at one end thereof to a particular cover which is still in free condition and in this condition the position of the other end of the spiral bimetal element relative to the cover is marked on a proper peripheral portion of the cover by paint or the like. In the subsequent assembling process the cover is mounted to the casing while its rotational position relative to the casing is adjusted depending upon the aforementioned guidemark by employing a cover mounting means as mentioned above which mounts the cover to the casing while permitting the cover to be adjusted relative to the casing with respect to its rotational position around the axis of the choke valve shaft. However, this conventional method is liable to cause incorrect mounting of the cover to the casing due to misreading of the indication mark. Furthermore, this conventional method of adjustment does not guarantee that the initial adjustment of the spiral bimetal element is maintained without being changed by the user of the automobile or by an inexperienced engineer during maintenance.